While selenium is an essential element for animals, toxicity may occur with as little as 5 mg/kg. Exposure to toxic levels manifests in birds and fish as embryo mortality and deformities, and poor post hatching survival. Selenium in the environment of these species may result from mining operations, for example, discharge from tailings impoundments, run-off from waste rock piles, discharge from fly ash ponds at fossil fuel combustion plants, or from impoundments or run-off from large scale agricultural irrigation.
Regulatory guidelines for the concentration level in North America for selenium discharge requirements are presently low and can be expected to trend lower. The USEPA has set the Maximum Contaminant Level (MCL) and the Maximum Contaminant Level Goal (MCLG) in drinking water for selenium at 0.05 mg/L. EPA has found selenium to potentially cause the following health effects when people are exposed to it at levels above the MCL for relatively short periods of time; hair and fingernail changes; damage to the peripheral nervous system; fatigue and irritability.
To meet future requirements, industrial and other discharging entities should plan on requirements of the order of 1-5 μg/L (ppb). Selenium discharges at this level are challenging because selenium exists in a variety of different forms, is usually at a dilute concentration, and treatment results in a concentrated residual which has to be disposed of without re-release of selenium.
The need for selenium removal or reduction technology has generated many approaches to this problem. These can be separated into four categories, each with their strengths and weaknesses. Practitioners desiring to select a technology will find that they face two daunting problems; the difference in effectiveness exhibited by the technologies on different selenium ion forms, selenate (Se+6 or SeVI) and selenite (Se+4 or SeIV), and other organocomplexes, such as selenocyanate, and the deleterious effect of other ions, particularly sulfate ions on the various technologies.
1. Standard Desalting Techniques                The use of reverse osmosis (RO) and nanofiltration (NF) to remove selenium from water has been reported. Nanofiltration can remove selenate, but is less effective against selenite. Reverse osmosis is reported able to remove selenate and selenite to less than about 5 μg/L at full scale.        Ion exchange (IX) can remove selenate but is less effective for selenite. Sulfate which has almost equivalent ion exchange affinity decreases the effectiveness of IX for selenate. It has been reported that arsenic removal media, such as DOW Adsorbsia™ removes selenite, but not selenate.        
2. Adsorption Techniques                Ferrihydrite (ferric oxyhydroxide mineral) precipitation, which may be used as a co-precipitate with ferric salts, effectively removes selenite, Se(IV) at pH<˜8, but is not effective for selenate, Se(VI). Reduction of Se(VI) prior to adsorption is required. The presence of other aqueous species in the solution may influence the removal of Se(IV)        Activated alumina adsorbs selenite at pH levels between 3-8. Aqueous silica adsorbs in preference to selenite at pH 7 but is no problem at pH 4, but selenate adsorption by alumina is poor. Selenate adsorption drops off rapidly with increasing pH and is less than 50% at pH 7. Sulfate and carbonate adsorption significantly interferes with selenate adsorption.        
3. Microbiological Processes                These are specific for selenate. The reaction residence time is hours, necessitating retention of large volumes of water or wastewater being treated in bioreactors. Nitrates and sulfates reduce effectiveness of this technique and must be removed or mitigated.        U.S. Pat. No. 4,519,913 describes a microbiological process that reduces the concentration of selenium ions in a waste solution by passing said waste solution through a treatment zone containing a porous matrix on which are retained populations of at least one bacteria of the genus Clostridium under anaerobic conditions, said bacteria being capable of metabolically reducing said selenium ions to water insoluble selenium metal. The water insoluble selenium metal resulting from this metabolic reduction is retained on the porous matrix and the resulting aqueous effluent has a lower water soluble selenium ion concentration.        U.S. Pat. No. 4,725,357 describes a method of removing dissolved hexavalent selenium from water by treating the selenium-containing water in a reactor containing microbial biomass and a nutrient for the biomass, substantially in the absence of free oxygen, to cause at least part of the selenium to be captured by particles having a size of 0.1 micron or greater; and passing the discharge from the reactor through a filter in order to filter out particles which captured the selenium. This method is suited for removing dissolved hexavalent selenium from water which contains a higher weight concentration of nitrate than of hexavalent selenium (measured as selenium). In such a process, the concentration of nitrate in the water is lowered to 5 mg/l or below, typically 2 mg/l or less.        In U.S. Pat. No. 5,271,831 a process for removing oxyanions of selenium by selenate respiring microorganisms may be obtained by reducing the nitrate concentration well below 1 mM. In this process, the required lowering of the nitrate concentration in selenium- and nitrate-containing waste water may be accomplished by employing a nitrate utilizing biomass under aerobic conditions in a first treatment zone to remove nitrate followed by a second treatment zone where an anaerobic microbiological reaction using selenate respiring microorganisms to affect the biological reduction of oxyanions of selenium to elemental selenium.        
4. Chemical Reduction Processes                These processes reduce selenate to selenite or selenium, and flocculate and co-precipitate the selenium ions or metal for collection and disposal. Ferrous, aluminum and zinc salts are used with ferrous salts being the most common. Iron metal is used sometimes with copper catalyst to reduce selenium ions to selenium metal which precipitates on the iron or as an insoluble iron selenite with ferric hydroxide formed by simultaneous oxidation.        U.S. Pat. No. 4,405,464 describes a method to substantially reduce the concentration of selenium ions in the selenate oxidation state in an aqueous solution by contacting the aqueous solution with metallic iron. The metal iron reduces selenium ions in the Se(VI) oxidation state to at least the Se(IV) oxidation state, and the metallic iron is oxidized and hydrolyzed to form a ferric hydroxide precipitate. The inventors of '464 believe that the selenium is either precipitated on the iron by a cementation process or precipitated on the ferric hydroxide by adsorption of the reduced selenite ions upon the surface of the precipitate to form an insoluble iron selenite.        U.S. Pat. No. 4,806,264 uses ferrous hydroxide at pH's between 8 and 10, preferably at about pH 9. Under these conditions, ferrous hydroxide reduces the selenium ions in an aqueous solution to elemental selenium and is itself oxidized to ferric oxides which are highly magnetic (magnetite and maghemite). The elemental selenium particles remain within the particles of the iron oxides and are collected and removed from the solution by magnetic means.        In U.S. Pat. No. 5,993,667 selenium is removed from selenium-containing water in a two stage process. The water is first cooled to approximately 80 to 90 degrees Fahrenheit and fed to a continuously stirred tank reactor where it is mixed with an aqueous solution of ferric sulfate or other soluble ferric salt to reduce the pH of the water and to produce a precipitate consisting of ferric hydroxide and ferric oxyhydroxide. In a second continuously stirred tank reactor, the treated water is mixed with an aqueous permanganate solution, causing the oxidation of the selenium to selenite and forming a manganese dioxide precipitate. The selenite is adsorbed on both the manganese dioxide and the ferric hydroxide, and is removed with them by centrifugation.        
The Selenium Workgroup of The North American Metals Council (http://www.namc.org/selenium.html) has published a report (http://namc.org/docs/00062756.PDF) which extensively reviews the present state of selenium removal technology. They state;
“While the physical, chemical and biological treatment technologies have the potential to remove selenium, there are few technologies that have successfully and/or consistently removed selenium in water to less than 5 μg/L at any scale. There are still fewer technologies that have been demonstrated at full scale to remove selenium to less than 5 μg/L, or have been in full scale operation for sufficient time to determine the long-term feasibility of the selenium removal technology. No single technology has been demonstrated at full scale to cost-effectively remove selenium to 5 μg/L for waters associated with all sectors. Therefore, performance of the technology must be demonstrated on a case specific basis.”
The inventors have realized that to economically and efficiently meet selenium removal requirements for the various cases that will arise will require a flexibly designed and integrated process scheme. The inventors describe herein a process that will treat and recycle the input selenium containing water stream, discharging the major part of input selenium containing water as treated water that meets the local requirements for release. Local requirement means the discharge concentration set by one or more local, state or federal governmental agencies, or requirements of downstream processes to which the discharge is sent.